The function of activated Hageman factor has been shown to be augmented by high molecular weight (HMW) kininogen as a prerequisite for significant conversion of prekallikrein to kallikrein in plasma. Kallikrein then enzymatically activates Hageman factor and this interaction is facilitated by HMW kininogen. Prekallikrein and HMW kininogen circulate in plasma as a non-covalently linked complex. Activated Hageman factor can be slowly inhibited by antithrombin III, however addition of heparin activates antithrombin and inhibition is then virtually instantaneous. Histamine was shown to enhance eosinophil migration to other chemotactic agents via the H-1 receptor and at higher doses, inhibits eosinophil migration by stimulating H-2 receptors. The inhibition appears to be secondary to raising the cellular level of cyclic AMP. The chemotactic activity of histamine itself is not inhibited by H-1 or H-2 receptor antagonists and may indicate the presence of a new type of histamine receptor.